I recently got a 2004 40th anniversary GT. I am told they came stock with a 65mm throttle body. I wanted to add a 75mm throttle body (and new intake plenum) but a coworker told me it would bog down because the 75mm was for modified motors. The only upgrades I currently have are a K & N intake, Flowmaster mufflers, and a Superchips microtuner (which I have flashed high-performance using 93 octane.) Should I go with a 70mm instead?
I'm on a budget and can only do a project a quarter but I plan to add a MSD 6AL ignition box, hi-flow cats, and headers this year. Any suggestions on which should be done first? Any other suggestions on biggest bang for the buck for performance would be appreciated.
For handling I plan to add Steeda sport springs. (Do I need anything esle to be able to add these?  

i would recommend doing the throttle body, cats, and headers at the same time, then getting your comp re-flashed to take advantage of the upgrades. a larger throttle body can only be an advantage if you upgrade the intake, but it won't neccesarily hurt you if you don't do any other upgrades. you do, however, need to upgrade your computer. you can upgrade the ignition without any affect to your comp flash, but it's not a great investment unless you plan on doing a turbo or supercharger anytime soon. instead of new ignition, how about a larger bore mass air meter? that, combined with the larger throttle body, would greatly increase the air flow to your motor. combined with the less restrictive exhaust, you should see a difference. then move on to a new f.i. manifold, then heads, then cam...then buy a bullet-proof short block and get crazy!  

Thanks for the info. If I can only do one at a time will the 75mm throttle body and larger plenum cause it to bog down?